AIR travellers can expect to be monitored, probed — and hopefully protected — by a handful of new security technologies, from chemical-sniffing detectors to computer cameras that scan crowds.

The devices could be installed in relatively short order at airports and on planes, aiming to keep hijackers and weapons from getting on board, and disarming those who do, experts say.

Fliers could pay for the measures with a $US30 ($58.95) to $US50 a ticket surcharge, one expert estimated.

Agencies that handle security at American airports, including the Federal Aviation Administration, US Customs Service and the US Immigration and Naturalisation Service, have little to say about technology upgrades under consideration.

But experts point to a combination of devices that screen luggage, clothes and criminal backgrounds.

The first line of defence is aimed at keeping weapons and explosives off planes, said Rick Charles, who heads the aviation program at Georgia State University.

Travellers can expect more frequent searches of luggage, documents and clothes with particle detectors that can find traces of drugs or explosives and, in some cases, pathogens used in biological warfare, he said.

Authorities will also probably call for more explosives-detecting resonance scanners that can hone in on tough-to-find plastic explosives, said Billie Vincent, president of Aerospace Services International and the FAA's former head of security.

Machines that can find bombs and guns provide no defence against passengers bent on destruction by other means — such as the suicide-minded groups that hijacked the four airliners on September 11.